In recent years, research related to a light emitting device having an EL element as a self-luminous element has been actively carried out. Particularly, a light emitting device using an organic material as an EL material has been attracting attention. The light emitting device is also referred to as an EL display device. The EL element includes a layer containing an organic compound, which generates luminescence (electroluminescence) by being applied with an electric field (hereinafter, referred to as an EL layer); an anode; and a cathode. The luminescence generated in the layer containing the organic compound includes luminescence (fluorescence) that is generated upon returning of electrons to a ground state from excited singlet state and luminescence (phosphorescence) that is generated upon returning of electrons to a ground state from excited triplet state.
The EL element has a structure in which an EL layer is sandwiched between a pair of electrodes. The EL layer generally has a lamination structure. Typically, a lamination structure of “a hole transporting layer, a light emitting layer, and an electron transporting layer” is cited. The structure provides greatly high light-emitting efficiency and has been used in almost all light emitting devices that have been researched and developed now.
Alternatively, a structure composed by sequentially laminating a hole injecting layer, a hole transporting layer, a light emitting layer, and an electron transporting layer on an anode or a structure composed by sequentially laminating a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injecting layer is also applicable. Fluorescent pigments and the like may be doped to the light emitting layer.
Either a low molecular weight material or a high molecular weight material can be used for forming these layers.
In addition, the EL layer is a generic term used to refer to all layers formed between a cathode and an anode. Therefore, all of each the above-mentioned hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer, and electron injecting layer are included in the EL layer.
Further, a light emitting element composed of a cathode, an EL layer, and an anode is referred to as an EL element. There are two kinds for forming the EL element as follows: a system for forming an EL layer between two kinds of striped electrodes that run at right angles to one another (passive matrix system); and another system for forming an EL layer between a pixel electrode and a counter electrode arranged in matrix that are connected to a TFT (active matrix system). When the pixel density is increased, it has been considered that the active matrix system has an advantage over the simple matrix system since the active matrix can be driven at lower voltage for having switches in each pixel (or each dot).
Since the EL element is extremely and easily deteriorated by being oxidized or absorbing moisture due to existence of oxygen or moisture, there has been a problem that the light-emitting efficiency of the EL element is decreased or the lifetime thereof is shortened. Therefore, moisture and oxygen has been conventionally prevented from penetrating into the EL element as follows: the EL element is covered with an opposing substrate, dry air is filled thereinto, and a drying agent is further attached thereto. A substrate with the EL element formed thereon and the opposing substrate are adhered to each other with a sealing agent (for example, see Patent Document 1).
A step for adhering the substrate with the EL element formed thereon and the opposing substrate with the sealing agent is referred to as a sealing or a sealing step.
Further, since an EL material is damaged by UV irradiation, there has been a problem in which the light-emitting efficiency of a light emitting element is reduced and lifetime thereof is shortened.
The EL element has conventionally included a structure in which an electrode is formed as an anode over a substrate, an organic compound layer is formed on the anode, and a cathode is formed on the organic compound layer so that light generated in the organic compound layer is emitted toward a TFT through the anode, which is a transparent electrode (hereinafter, the structure is referred to as a bottom emission structure).    [Patent Document 1]: Japanese Patent Application Laid-Open No. 2002-352951
The EL element can be covered with the opposing substrate in the above-mentioned bottom emission structure. However, in the case of a structure in which an electrode is formed as an anode over a substrate, a layer containing an organic compound is formed on the anode, and a cathode that is a transparent electrode is formed on the layer containing the organic compound (hereinafter, the structure is referred to as a top emission structure), an opposing substrate made from a light shielding material cannot be used. Similarly, the same is true in the case of a dual emission structure in which light is simultaneously emitted upward and downward. As compared with the bottom emission structure, in the top emission structure and the dual emission structure, the number of material layers through which light emitted from the layer containing the organic compound passes can be reduced, thereby suppressing stray light generation between the material layers having different refractive indices. In the case of the bottom emission structure, it is necessary to pay minute attention to handling of a drying agent so as not to absorb moisture. Therefore, it has been necessary to encapsulate the drying agent quickly. Further, in the case of the top emission structure and the dual emission structure, when the drying agent is arranged on a pixel portion, the drying agent hinders display.
Further, the substrate with the EL element formed thereon and the opposing substrate are attached to each other with an UV-curable or a heat-curable sealing agent, wherein the EL element exists inside a space hermetically-sealed with the sealing agent, the opposing substrate, and the substrate. It is preferable that moisture and oxygen do not exist in the space and do not penetrate thereinto. When oxygen and moisture exist therein, a problem in which the EL element is deteriorated has been caused. The sealing agent is also referred to as a sealing material.
As compared with the heat-curable sealing agent, the UV-curable sealing agent is quickly cured using a device with smaller size, and therefore the UV-curable sealing agent has advantages in mass production. Therefore, there are many sealing devices for mass production each of which has only a function of UV irradiation as a function of curing the sealing agent. In the case of using such sealing devices, however, the heat-curable sealing agent cannot be used therein.
Further, since the EL element is damaged by UV irradiation and thermal shock, there has been a problem of decreasing the luminance for the EL element and a problem of shortening its lifetime.
When sealing materials before being cured are in contact with each other for a long time, the sealing materials are likely to be mixed with each other, which result in deformation. Further, since a mixed portion of the mixed sealing materials is not cured uniformly, the adhesive strength is likely to be reduced.
In order to overcome the foregoing problems, it is an object of the present invention as disclosed in the specification is to provide a light emitting device having a structure for preventing oxygen and moisture from penetrating into an EL element, and a method of manufacturing the same. Furthermore, with respect to the top-emission structure and the dual-emission structure in addition to the bottom-emission structure, it is an object of the present invention to encapsulate an EL element by uniformly curing all of the sealing materials without inserting a drying agent and damaging the EL element due to the UV irradiation even when a sealing device, which only has a function of UV irradiation, is used.